The radiographic image such as an X-ray image has been used over an extensive range in the field of medical treatment for the diagnosis of the state or progress of a disease. Specifically, the radiographic image based on an intensifying screen-film combination has been improved to enhance sensitivity and image quality in its long history. As a result, it is still employed in the field of medical treatment all over the world as an image pickup system characterized by both a high degree of reliability and excellent cost/performance ratio. However, such image information pertains to so-called analog image information, which is not suited for free image processing or instantaneous transmission, unlike the digital image information that has been making a rapid progress in recent years.
In recent years, a radiographic image detecting apparatus of digital system represented by the Computed Radiography (CR) or flat panel detector (FPD) is coming on the market. The apparatus of this type directly provides a digital radiographic image, and directly shows an image on an image display apparatus such as a cathode ray tube or liquid crystal panel. It does not always require formation of an image on a photographic film. Thus, the X-ray image detecting apparatus of the digital system reduces the need of forming an image by silver halide photography, and hence provides a substantial improvement in the convenience of diagnostic operation in a hospital or clinic.
The Computed Radiography (CR) as one of the X-ray image digital techniques is currently being accepted in the field of medical treatment. However, it is insufficient in sharpness of the image and spatial resolution, and has not yet reached the image quality level of the screen/film system. A flat panel X-ray detector (FPD) using a thin film transistor (TFT) has been developed as a newer digital X-ray image technique, as disclosed, for example, in an “Amorphous Semiconductor Usher in Digital X-ray Imaging” by John Rowlands, “Physics Today”, November 1997, P. 24, or “Development of a High Resolution Active Matrix, Flat-Panel Imager with Enhanced Fill Factor” by L. E. Antonuque, SPIE, P. 2. Vol. 32.
A scintillator plate containing an X-ray phosphor which emits light when irradiated with X-rays is used to convert radiation into visible light. To improve the SN ratio in low-dose photographing, it is necessary to use a scintillator plate of high light emitting efficiency. Generally, the emitting efficiency of a scintillator plate is determined by the thickness of a phosphor layer and the X-ray absorbency coefficient of the phosphor. However, increase in the thickness of the phosphor layer results in scattering of the light emitted in the phosphor layer, whereby the sharpness of the image is reduced. Accordingly, the film thickness is determined when the sharpness required for image quality is determined.
Cesium iodide (CsI) is characterized by a relatively high conversion ratio of X-rays to the visible light, and is capable of easily forming a phosphor in a columnar crystal structure by evaporation. Thus, the scattering of the light emitted in the columnar crystal is reduced due to the light guiding effect, and the thickness of the phosphor layer can be increased. However, if only CsI is used, the light emitting efficiency is lower. As shown in Patent Document 1, CsI blended with sodium iodide (NaI) in a desired mole ratio is deposited on the substrate as sodium activated cesium iodide (CsI:Na) by vapor deposition. In recent years, CsI blended with thallium iodide (TlI) at a desired mole ratio is deposited on the substrate as thallium activated cerium iodide (CsI:Tl). This is annealed in the post-process to improve the visible light conversion efficiency and the resulting product is used as an X-ray phosphor.
However, the CsI-based scintillator (phosphor layer) is hygroscopic and has a problem of being deteriorated with the lapse of time. To avoid such deterioration with the lapse of time, methods are proposed to form a moisture proof protective layer on the surface of the CsI-based scintillator (phosphor layer). For example, in one of such methods, the top and side of the scintillator layer (corresponding to the phosphor layer of the present invention) and the outer peripheral portion of the scintillator layer of the substrate is covered with a polyparaxylylene resin (e.g., Patent Document 2). However, the polyparaxylylene resin disclosed in the Patent Document 2 is less moisture-proof and is incapable of providing a sufficient protection of the phosphor layer. Further, the polyparaxylylene resin enters the gap of the columnar crystal constituting the scintillator layer, whereby the light guiding effect is hindered.
There is a method known in the conventional art wherein a transparent resin film having a moisture permeation of less than 1.2 g/m2/day is used to cover at least the side opposite the supporting member of the scintillator layer as well as the lateral side thereof (e.g., Patent Document 3). However, according to the method described in Patent Document 3, when a transparent organic polymer film such as polypropylene or polyethylene terephthalate is placed as a protective layer in a state of being coherent (or in close contact) with the top of the phosphor layer, a high moisture proof property can be obtained, but the sharpness of the image is reduced, which is a serious defect. To avoid this, the film thickness must be kept at 5 μm or less, which is insufficient to protect the phosphor layer against chemical degeneration or physical impact. Thus, this material is not fully suitable as a protective layer.
For the reasons mentioned above, there has been a demand for development of a scintillator panel sealed by a protective film capable of avoiding deterioration of the phosphor layer with the lapse of time, and protecting the phosphor layer from chemical deterioration and physical impact, while preventing deterioration in the sharpness of the image.
Patent Document 1: Examined Japanese Patent Publication No. 54-35060
Patent Document 2: Unexamined Japanese Patent Application Publication (hereinafter referred to as JP-A) No. 2000-284053
Patent Document 3: JP-A No. 2005-308582